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设计神经网络的一般步骤：

1. 设计框架

2. 设计骨干网络

Unet网络设计的步骤：

1. 设计Unet网络工厂模式

2. 设计编解码结构

3. 设计卷积模块

4. unet实例模块

Unet网络最重要的特征：

1. 编解码结构。

2. 解码结构，比FCN更加完善，采用连接方式。

3. 本质是一个框架，编码部分可以使用很多图像分类网络。

示例代码：

import torch
import torch.nn as nn

class Unet(nn.Module):
 #初始化参数：Encoder,Decoder,bridge
 #bridge默认值为无，如果有参数传入，则用该参数替换None
 def __init__(self,Encoder,Decoder,bridge = None):
  super(Unet,self).__init__()
  self.encoder = Encoder(encoder_blocks)
  self.decoder = Decoder(decoder_blocks)
  self.bridge = bridge
 def forward(self,x):
  res = self.encoder(x)
  out,skip = res[0],res[1,:]
  if bridge is not None:
   out = bridge(out)
  out = self.decoder(out,skip)
  return out
#设计编码模块
class Encoder(nn.Module):
 def __init__(self,blocks):
  super(Encoder,self).__init__()
  #assert:断言函数，避免出现参数错误
  assert len(blocks) > 0
  #nn.Modulelist（）：模型列表，所有的参数可以纳入网络，但是没有forward函数
  self.blocks = nn.Modulelist(blocks)
 def forward(self,x):
  skip = []
  for i in range(len(self.blocks) - 1):
   x = self.blocks[i](x)
   skip.append(x)
  res = [self.block[i+1](x)]
  #列表之间可以通过+号拼接
  res += skip
  return res
#设计Decoder模块
class Decoder(nn.Module):
 def __init__(self,blocks):
  super(Decoder, self).__init__()
  assert len(blocks) > 0
  self.blocks = nn.Modulelist(blocks)
 def ceter_crop(self,skips,x):
  _,_,height1,width2 = skips.shape()
  _,_,height2,width3 = x.shape()
  #对图像进行剪切处理，拼接的时候保持对应size参数一致
  ht,wt = min(height1,height2),min(width2,width3)
  dh2 = (height1 - height2)//2 if height1 > height2 else 0
  dw1 = (width2 - width3)//2 if width2 > width3 else 0
  dh3 = (height2 - height1)//2 if height2 > height1 else 0
  dw2 = (width3 - width2)//2 if width3 > width2 else 0
  return skips[:,:,dh2:(dh2 + ht),dw1:(dw1 + wt)],\
    x[:,:,dh3:(dh3 + ht),dw2 : (dw2 + wt)]

 def forward(self, skips,x,reverse_skips = True):
  assert len(skips) == len(blocks) - 1
  if reverse_skips is True:
   skips = skips[: : -1]
  x = self.blocks[0](x)
  for i in range(1, len(self.blocks)):
   skip = skips[i-1]
   x = torch.cat(skip,x,1)
   x = self.blocks[i](x)
  return x
#定义了一个卷积block
def unet_convs(in_channels,out_channels,padding = 0):
 #nn.Sequential：与Modulelist相比，包含了forward函数
 return nn.Sequential(
  nn.Conv2d(in_channels, out_channels, kernal_size = 3, padding = padding, bias = False),
  nn.BatchNorm2d(outchannels),
  nn.ReLU(inplace = True),
  nn.Conv2d(in_channels, out_channels, kernal_size=3, padding=padding, bias=False),
  nn.BatchNorm2d(outchannels),
  nn.ReLU(inplace=True),
 )
#实例化Unet模型
def unet(in_channels,out_channels):
 encoder_blocks = [unet_convs(in_channels, 64),\
      nn.Sequential(nn.Maxpool2d(kernal_size = 2, stride = 2, ceil_mode = True),\
         unet_convs(64,128)), \
      nn.Sequential(nn.Maxpool2d(kernal_size=2, stride=2, ceil_mode=True), \
         unet_convs(128, 256)),
      nn.Sequential(nn.Maxpool2d(kernal_size=2, stride=2, ceil_mode=True), \
         unet_convs(256, 512)),
      ]
 bridge = nn.Sequential(unet_convs(512, 1024))
 decoder_blocks = [nn.conTranpose2d(1024, 512), \
      nn.Sequential(unet_convs(1024, 512),
         nn.conTranpose2d(512, 256)),\
      nn.Sequential(unet_convs(512, 256),
         nn.conTranpose2d(256, 128)), \
      nn.Sequential(unet_convs(512, 256),
         nn.conTranpose2d(256, 128)), \
      nn.Sequential(unet_convs(256, 128),
         nn.conTranpose2d(128, 64))
      ]
 return Unet(encoder_blocks,decoder_blocks,bridge)